Wavefront errors can be described and represented mathematically, for instance, by the so-called Zernike polynomials. They are detectable, for example, by wavefront aberrometers. In this case, the wavefront errors may be caused by individual imaging errors or by superposition of different imaging errors of single or multiple optical elements of an optical system. In many cases, individual optical elements have a dominant influence on the relevant imaging errors of an optical system, and the wavefront deformation caused thereby has a typical appearance, which can be countered by active deformation of the surface of one of the optical elements or, for example, of a mirror additionally arranged in the system for this purpose.
In order to counter imaging errors, a large number of solutions are known from the prior art which use active surface deformation of a planar optical element, usually a mirror. They differ substantially in the location where an actuating means acts on the planar optical element and where, accordingly, forces are applied into the planar optical element. In some of these solutions, forces are applied on the peripheral surface of the planar optical element or in the edge region adjacent to the peripheral surface. Only such solutions will be considered below.
Using a correction device in accordance with US 2012/0275041 A1, an imaging error of known evolution can be corrected within an optical instrument. For this purpose, a deformable mirror is introduced into the optical path of the arrangement, along which beams of electromagnetic radiation propagate. Forces are applied to the peripheral edge of the deformable mirror and introduced into the mirror such that the latter is deformed as a function of its contour as well as the location of force introduction and the vectors (amount, direction) of the forces introduced. The deformation of the mirror caused thereby and the resulting local change in the reflection behavior, as a consequence of optical path differences, enable correction of any wavefront errors occurring.
Specifically, the aforementioned US 2012/0275041 A1 proposes to select the contour of the mirror as a function of the imaging error to be corrected, e.g. round so as to correct the focus position, or elliptical so as to correct the focus position and astigmatism. Despite the same forces acting on the perimeter, this allows locally differing flexing moments to be applied to the surface, depending on the distance of the force application from the center of the mirror. It is proposed therein that, as a means of force application, an intermediate plate having the same contour as the mirror be connected to the peripheral edge of the mirror by a ring and that a force be introduced centrally into the intermediate plate, e.g. by means of a piezo actuator, said force then acting on the periphery of the mirror. In order to differentiate the force effect along the periphery of the mirror, it is proposed to design the intermediate plate differentially in thickness or/and to introduce the forces eccentrically into the intermediate plate. This solution allows different flexing moments to be generated in the mirror along the periphery. However, the ratio of the flexing moments with respect to each other is predetermined by the design of the contour of the mirror and is thus no longer variable.
U.S. Pat. No. 7,229,178 B1 discloses a deformable mirror whose circular or oval mirror plate can be bent parabolically via an internal (smaller) and an external (larger) annular support. For this purpose, the mirror is placed between the annular support. The required force is introduced by a mechanical actuating element. The actuating element acts directly or indirectly, via a lever, on the annular supports. Force introduction via the level and parallel annular supports is always effected centrally with respect to the axis of symmetry of the mirror. This merely allows the parameters of a paraboloid surface to be influenced. Again, no locally differentiated application of force is possible.
In a device known from DE 601 16 322 322 T2, aberrations are also compensated for by application of force onto a mirror. For the application of force, there are provided at least one active actuating element and two so-called force transmission rods. The force transmission rods each have their ends connected to the mirror via a respective deformation element, thereby applying the same force into the mirror at different points. A differentiated application of force at different points is not possible.
DE 196 28 672 C2 discloses a mirror comprising a deformable mirror plate held by its edge as well as comprising an actuating mechanism acting on the rear surface of the mirror plate, said actuating mechanism comprising a plurality of springs connected in parallel, which act on the rear surface of the mirror plate, and comprising an actuating means for adjustment of the spring force. In this case, a force is applied into the center of the mirror and into eccentric points of action which are arranged in radially opposite pairs. The introduction of force can be adjusted only jointly for all points of action.
DE 10 2007 010 906 A1 proposes a device with an actuating means, comprising at least one main actuator which acts on the peripheral surface of an optical element. In one embodiment, a bending moment is introduced into the optical element, with the axis of the bending moment being perpendicular to the optical axis and perpendicular to a radial direction. In the case of only one main actuator, the optical element is fixed on the opposite side in the axial direction. Advantageously, main actuators are provided on two opposite sides with respect to the optical axis, each of said main actuators introducing a respective bending moment into the optical element. As a main actuator, an electrodynamic drive is proposed. In particular, an electromagnetic plunger coil drive is supposed to be suitable to introduce high forces with great accuracy of repetition and great positioning accuracy so as to bend the optical element in a very accurate and reproducible manner. The device is suitable, in particular for dynamic correction of wavefront errors, for example those of a holographic projection device, and are not suitable for static correction.
U.S. Pat. No. 3,601,343 A discloses a mount for strain-free support of an element with an expansion coefficient that differs from that of the mount. The mount consists of a rigid annular body having a plurality of flexible support arms formed thereon in an axial direction of the mount, which grip the element to be supported at their free ends. The mount merely allows a radial movement and thus unhindered, different expansion of the mount and the element in a radial direction.
DE 198 27 603 A1 discloses, in connection with a projection lighting system of microlithography, a mount in which an optical element is arranged, and actuators which act on the optical element perpendicular to the optical axis. The actuators are designed to cause flexing of the optical element without substantial changes in thickness.
DE 10 2012 209 309 A1 discloses a lithography system comprising at least one mirror assembly. A mirror assembly of lithography systems comprises mirrors with a large diameter, which makes low-deformation support and actuation difficult. According to the aforementioned DE 10 2012 209 309 A1, the mirror is formed from a mirror substrate, whose front surface is provided with a reflective surface, and an annular side wall, said mirror substrate and said side wall limiting a hollow space, resulting in the mirror having a comparatively low weight. The mirror is supported, actively or passively, on a structural element of the lithography system by a plurality of supporting elements via the side wall. This results in deformation being uncoupled to a large extent to the reflective surface of the mirror.